If you tend to have a large number of tabs open at a time, get ready to experiment considerably faster session restores thanks to cascaded session restore (CSR), an optimized approach that loads a limited number of tabs at a time (3 by default) when Firefox restarts.
In recent Firefox 4 nightlies, it prioritizes the currently viewed and visible tabs as they are most likely what you want to see first. Favicons and titles are loaded for all tabs, and if you switch to a tab that is not loaded yet, it is loaded immediately.
You can customize the number of tabs Firefox can restore at time setting browser.sessionstore.max_concurrent_tabs to the preferred value via about:config.
The idea is based on BarTab, a Firefox extension that does just this for current versions of Firefox.